Sports supplement firm USPlabs says it has stopped using an ingredient flagged by federal regulators and has destroyed its inventory of a popular product linked to dozens of liver injury cases.

Although USPlabs says it "knows of no valid concern" about the safety of its OxyElite Pro weight loss supplement or an ingredient called aegeline, negative publicity has prompted the company to discontinue using the ingredient in the United States, according to a letter the Dallas-based company sent this week to the U.S. Food and Drug Administration.

"As FDA is aware, the Company has voluntarily destroyed its distribution center inventory of all dietary supplements containing aegeline. Reformulated replacement products will be marketed shortly," said the letter written by USPlabs chief compliance officer Kenneth Miles. The company, which provided the letter to USA TODAY, said it had no further comment.

The 16-page letter dated Monday was in response to the FDA issuing a formal warning to USPlabs on Oct. 11 accusing the company of improperly using aegeline in OxyElite Pro and another supplement, VERSA-1. The agency said it was not aware of any information demonstrating that aegeline was lawfully marketed as a dietary ingredient in the U.S. before October 1994, a key regulatory date for grandfathering supplement ingredients.

The FDA has advised consumers not to use OxyElite Pro and VERSA-1 while the outbreak investigation is ongoing. Federal health officials have not directly implicated aegeline as the suspected ingredient in the outbreak. But the FDA's warning letter said that establishing that aegeline products are safe "is particularly important" given the liver injury cases linked to OxyElite Pro in Hawaii and other states. FDA said several findings "suggest a causal connection may exist" between the illness reported in Hawaii and consumers taking OxyElite Pro.

The FDA had no additional information to provide about the outbreak investigation, a spokeswoman said.

As of Wednesday, the Centers for Disease Control and Prevention said it had confirmed 56 cases in the outbreak. There has been one death and some cases required liver transplants among people sickened since April 1, officials have said. The vast majority of the cases — at least 43 — have been identified in Hawaii, which has a top health official in that state worried that doctors in mainland states haven't reported their cases to health authorities.

"I am concerned that it's not really in the awareness of the public and clinicians on the mainland," said Sarah Park, Hawaii's state epidemiologist, whose department in September was the first to identify a cluster of liver failure cases among otherwise healthy dietary supplement users.

Park said that while her state's investigation shows a strong association between OxyElite Pro and people who suffered liver problems, "we don't know if this aegeline is the actual issue or not." She said the cases appear to be associated with a newly formulated version of OxyElite Pro, which includes aegeline among other listed ingredients. USPlabs put the aegeline version on the market after the FDA pressured the company to stop using the controversial stimulant DMAA earlier this year.

In its letter this week to the FDA, USPlabs said that the disproportionate number of cases in Hawaii and the lack of cases elsewhere "conclusively shows that neither OxyElite Pro or its ingredients are causing the epidemic in Hawaii." The company says that if its product were really at fault, there should be more than 9,000 cases on the mainland, based on the number identified in Hawaii.

Park said Hawaii's small and contained population created unique circumstances that has made it easier to identify cases that may appear isolated on the mainland. Hawaii has only one liver transplant center, she said, and flags were raised by doctors at the hospital who encountered five otherwise healthy people developing severe hepatitis and needing to be evaluated for possible transplants. Concentrated media attention that blanketed the whole state resulted in doctors reporting dozens of other less severe liver injury cases that they'd successfully treated, she said.

"I'm highly suspicious there are more cases on the mainland than the ones already being investigated by the CDC," Park said.

Park said many doctors don't know to report adverse events involving dietary supplements to the FDA. She noted that none of the initial severe hepatitis cases evaluated in Hawaii for possible liver transplants had been reported to the FDA's MedWatch program even though dietary supplement use was suspected.

USPlabs' elimination of aegeline marks the second time the company has reformulated OxyElite Pro in recent years. In July, under pressure from the FDA, USPlabs destroyed $8 million worth of OxyElite Pro and Jack3d supplements that contained DMAA. While USPlabs had maintained that the stimulant DMAA was all-natural and derived from the geranium plant, the FDA said it wasn't natural and warned that products containing the compound posed health risks.